1. Field of the Invention
This invention relates to an etching liquid, an etching method, and a method of manufacturing an electronic component, and more particularly to an etching liquid, an etching method, and a method of manufacturing an electronic component where silicon nitride can be selectively etched.
2. Background Art
The process of manufacturing electronic components such as semiconductor device, liquid crystal displays, and phase shift masks includes wet etching for selectively removing silicon nitride film. For example, regarding semiconductor device, a nitride film (silicon nitride film, Si3N4) is used as a hard mask for forming a device isolation oxide film (silicon oxide film, SiO2 film) on a wafer. To remove such nitride film, wet etching with heated phosphoric acid solution, called the hot phosphoric acid process, is typically used. The hot phosphoric acid solution (etching liquid) has the property of having a higher etching rate of silicon nitride film (Si3N4) relative to that of silicon oxide film (SiO2 film). Hence it has been used for selectively removing a hard mask made of silicon nitride film (Si3N4) without removing device isolation oxide film made of silicon oxide film (SiO2 film).
With the recent demand for high integration and downscaling, the device isolation structure of a semiconductor device is in transition from LOCOS (Local Oxidation of Silicon) structure to STI (Shallow Trench Isolation) structure. However, if the conventional wet etching with hot phosphoric acid solution (etching liquid) is used for the STI (Shallow Trench Isolation) structure, the device isolation oxide film (silicon oxide film, SiO2 film) is removed during removing the hard mask (silicon nitride film, Si3N4) because of the insufficient selection ratio. This may cause trouble in the shape and size of the wafer cross section in the vicinity of the trench.
Conventionally proposed etching liquids for wet etching include an etching liquid with increased etching rate of silicon nitride film relative to that of silicon oxide film (see JP 2000-058500A) and an etching liquid for etching oxide film and nitride semiconductor (see JP 2001-284314A).
However, in the technique disclosed in JP 2000-058500A, a silicon-containing organic compound dissolved in a solvent is added to a phosphoric acid solution. Thus, unfortunately, silicon added or dispersed in the phosphoric acid solution may be adsorbed or precipitated on the pattern of a wafer.
On the other hand, JP 2001-284314A relates to a technique for increasing the etching rate of sapphire (Al2O3) relative to that of nitride semiconductor. This etching liquid cannot etch silicon nitride film (Si3N4).